This invention is concerned with an instrument for measuring displacement of a body or object in a harsh environment such as is present in the vicinity of an underwater explosion. Instruments for measuring displacements due to explosive forces are, in the process often destroyed or rendered ineffective themselves.
There are three basic methods of measuring displacement: (1) mechanical, (e.g., deformable material, such as lead cones); (2) acoustic timing (e.g., transmit and receive); and, (3) accelerometers. Lead cones have limitations in their use in measuring single axis deflections. The lead cone is placed between a body or object to be moved and a rigid (fixed) structure. The cone collapses upon movement of the body or object toward the rigid structure. The disadvantage of the lead cone is that it measures only maximum displacement in one direction and cannot be re-used, and is subject to shock damage. Acoustic devices are basically pressure measuring devices. Shock waves may "blind" receivers during critical measuring times. Thus, acoustic devices are not ideal in explosive environments. Accelerometers deliver a signal proportional to the acceleration of the moving body (object). To determine single axis (linear) displacement requires double integration of the data with respect to time. During this double integration, errors tend to add and not average out. Small accelerations and timing errors lead to inaccurate displacement measurements.
In the setting where the present invention will be used, it will be desired to measure the direction, extent and rate of linear displacement of a body relative to a rigid (fixed) structure, for example. It is desirable to obtain these data in an explosive environment without experiencing destruction or malfunction of test equipment.